A spacecraft propulsion system is a subsystem of a spacecraft whose purpose is to change the state of motion of the spacecraft from its natural Keplerian motion. The Keplerian motion is due to the gravity field of the solar system bodies. Among the figures of merit of a propulsion system are the payload mass fraction and the delta-v the propulsion system can produce. Larger is better for both figures of merit. The payload mass fraction is the payload mass divided by the total initial mass (payload mass plus initial propulsion system mass) of the spacecraft. The delta-v is the time integral, computed over the working time of the propulsion system, of the non-gravitational acceleration provided by the propulsion system.
Conventional propulsion systems include chemical rockets and electric propulsion. For payload mass fraction of ⅓ the best chemical rocket bipropellant (liquid hydrogen plus liquid oxygen) provides a delta-v of about 1 AU/year, where AU means one astronomical unit, essentially equal to 150 million kilometres. Higher delta-v values are possible but with exponentially decreasing payload mass fraction. For electric propulsion systems a firmer delta-v value does not exist, but typical values for realized missions are 2-4 AU/year. These delta-v values are not sufficiently high for many purposes, e.g. for reaching outer solar system targets in a reasonable time. While somewhat higher delta-v values can be generated by reducing the payload mass fraction to a minimum, for a fixed payload this means an exponential growth of the initial mass and corresponding exponential increase of the mission cost.
For producing propulsive force alternative solutions exist that take advantage of naturally occurring phenomena in space. A solar sail is a large sheet of thin membrane that the spacecraft deploys once outside the Earth's atmosphere. Photons originating from the Sun hit the sail as a continuous stream, thus transferring momentum to it. A magnetic sail consists of one or more large-area loops of (preferably super conductive) wire, through which an electric current is driven in order to create a magnetic field. The field interacts dynamically with the solar wind and generates a propulsive force. A magnetic sail is known from the prior art publication R. M. Zubrin, D. G. Andrews: “Magnetic sails and interplanetary travel”, Journal of Spacecraft and Rockets, Vol. 28, No. 2, pp. 197-203, published in 1991.
The idea of using solar wind for generating propulsion is also known from the publication P. Janhunen: “Electric Sail for Spacecraft Propulsion”, Journal of Propulsion and Power, Vol. 20, No. 4, pp. 763-764, published in 2004 and incorporated herein by reference. Solar wind means the continuous stream of charged particles, mostly high-energy electrons and protons, that the Sun emits continuously to essentially all radial directions. An electric sail is an electrically conductive structure that is held at a positive potential with respect to the solar wind plasma. In contrast to the solar sail which extracts momentum from the Sun's electromagnetic radiation and not the solar wind, the electric sail does not need to be a continuous sheet. Said prior art publication presents an example in which the electric sail is a mesh of wires with a spacing less than or equal to the so-called Debye length of the plasma. The Debye length is a measure of the distance over which an Individual charged particle can exert an effect.
Although the usability of an electric sail for spacecraft propulsion has thus been theoretically shown, there are no known practical solutions that would be applicable to implement the principle on actual space missions.